This invention relates to an enrichment circuit for use in an electronic fuel injection system for controlling the injection of fuel into an internal combustion engine. More particularly, it relates to an enrichment circuit for this purpose which is utilized during cranking of the engine, and preferably for a relatively short time interval after starting of the engine, to increase the quality of fuel supplied to the engine beyond the quantity which would be supplied to the engine were it not being cranked or just started.
The enrichment circuit of the invention is particularly intended for use in an electronic fuel injection system of the type which utilizes an electromagnetic fuel injector or injectors operated intermittently so that the quantity of fuel supplied to the internal combustion engine is a function of the duration of electrical pulses applied to the injectors. Nevertheless, the enrichment circuit of the invention may be utilized with continuous or other types of electronic fuel injection systems which provide an electrical signal having a characteristic which is representative of the amount of fuel to be supplied to an engine. Prior art fuel injection systems which produce a fuel control electrical signal of this kind include those described in U.S. Pat. Nos. 3,741,171 to Dautel, 3,742,920 to Black, 3,747,575 to Eisele et al, 3,747,576 to Gordon et al, 3,747,577 to Mauch et al and 3,763,833 to Rachel. These patents are illustrative of electronic fuel injection systems in which the enrichment circuit teachings of the invention may be incorporated.
In prior art intermittent electronic fuel injection systems for internal combustion engines, the usual technique for controlling fuel injector pulse duration during cranking of the engine is to supply a fixed pulse width to the injectors. This fixed pulse width during cranking may vary as a function of engine temperature or other engine parameters, for example, the fixed pulse width may be of one value when the engine is warm and of a different value when the engine is cold. In any event, this fixed cranking pulse width of the prior art must be determined for each engine design through mapping of the characteristics of that engine so that a suitable pulse width function may be obtained to achieve engine starting. Usually, the fixed pulse width value utilized during cranking is somewhat lean with respect to the air-fuel ratio required to achieve starting of the engine, but the cranking of the engine over a period of time tends to increase the richness of the air-fuel mixture until the engine actually starts. This kind of prior art cranking system does not take into account the condition of the engine with regard to the quantity of fuel which may already be in its intake manifold or combustion chambers immediately prior to cranking.